DOL & Star-Delta Motor Starting Current Sizer
When an induction motor starts from standstill, it draws a severe locked-rotor current (typically 6 to 8 times Full Load Current under DOL). This inrush causes immediate voltage dips across the feeder cable and upstream switchboard, potentially tripping contactors or causing motor stall. This tool calculates terminal voltage drop and motor torque depression from first principles.
Physics Reminder: Induction motor starting torque is proportional to the square of terminal voltage (
T ∝ V²). A 15% voltage dip reduces available breakaway torque to just 72% of rated!Starting Dynamics & Voltage Dip
Full Load Current (FLC)28.2 A
Peak Inrush Current (Istart)197.4 A
Feeder Voltage Drop (ΔV)14.2 V
Terminal Voltage Dip3.5%
Terminal Voltage at Start385.8 V
Available Starting Torque93.1% of rated
Show Full Mathematical Derivation (AM2 Working Standard)BS 7671:2018+A4:2026
AM2 PORTFOLIO & DESIGN AUDIT FORMATTRANSPARENT FIRST-PRINCIPLES MATHS
Engineering Guidelines & Planning Limits
- ENA P28 Supply Flicker Limit: Typically 3% maximum instantaneous voltage change on public supplies to prevent lighting flicker.
- Star-Delta Transition Spike: Open-transition starters produce a momentary current spike approaching DOL magnitude during the contactor changeover from star to delta.
- Motor Feeder Drop: Maximum 10% to 12% at motor terminals during start to guarantee magnetic contactor coils do not drop out (typical coil drop-out threshold is 65% to 75% nominal).
- High Inertia Loads: Fans, centrifuges, and loaded conveyors require high starting torque. If voltage dips >12%, starting time extends significantly, risking overload relay trip on start.
Industrial Engineering Compliance Note:Worked example to BS EN 60034 and BS EN 60204-1. Sizing starting currents for marginal supplies or generator sets must be verified against actual manufacturer nameplate data before installation (EAWR 1989).